1. Field
The present disclosure relates to an automatic transmission.
2. Description of the Related Art
Hitherto, an automatic transmission capable of providing 8-forward-gear-speed and 1-reverse-gear-speed transmission using four planetary gear mechanisms and six engagement mechanisms including clutches and brakes is known (refer to, for example, Japanese Unexamined Patent Application Publication No. 2012-97864 (Patent Document 1)).
The automatic transmission in the Patent Document 1 includes an input shaft rotatably supported in a housing, and an output member including an output gear disposed concentrically with the input shaft in the housing. Rotation of the output member is transmitted to left and right driving wheels of a vehicle via a differential gear or a propeller shaft.
Four planetary mechanisms, that is, a first planetary gear mechanism to a fourth planetary gear mechanism are concentrically disposed with the input shaft in the housing. The first planetary gear mechanism is what is called a single-pinion planetary gear mechanism including three elements, that is, a first sun gear, a first ring gear, and a first carrier that supports a first pinion so that the first pinion is rotatable about its axis and revolvable. The first pinion engages with the first sun gear and the first ring gear. (The single-pinion planetary gear mechanism is also called a minus planetary gear mechanism or a negative planetary gear mechanism because when the carrier is fixed and the sun gear is rotated, the ring gear and the sun gear rotate in different directions.) When the ring gear is fixed and the sun gear is rotated, the carrier rotates in the same direction as the sun gear.)
When the three elements of the first planetary gear mechanism are defined as a first element, a second element, and a third element that are arranged from one side in the sequence shown in the collinear diagram of the first planetary gear mechanism (a diagram that can show the ratios of the relative rotational speeds among the three elements, the sun gear, the carrier, and the ring gear in straight lines (speed lines)), the first element is the first sun gear, the second element is the first carrier, and the third element is the first ring gear.
The second planetary gear mechanism is also what is called a single-pinion planetary gear mechanism including three elements, that is, a second sun gear, a second ring gear, and a second carrier that supports a second pinion so that the second pinion is rotatable about its axis and revolvable. The second pinion engages with the second sun gear and the second ring gear. When the three elements of the second planetary gear mechanism are defined as a fourth element, a fifth element, and a sixth element that are arranged from one side in the sequence shown in the collinear diagram of the second planetary gear mechanism, the fourth element is the second ring gear, the fifth element is the second carrier, and the sixth element is the second sun gear.
The third planetary gear mechanism is also what is called a single-pinion planetary gear mechanism including three elements, that is, a third sun gear, a third ring gear, and a third carrier that supports a third pinion so that the third pinion is rotatable about its axis and revolvable. The third pinion engages with the third sun gear and the third ring gear. When the three elements of the third planetary gear mechanism are defined as a seventh element, an eighth element, and a ninth element that are arranged from one side in the sequence shown in the collinear diagram of the third planetary gear mechanism, the seventh element is the third sun gear, the eighth element is the third carrier, and the ninth element is the third ring gear.
The fourth planetary gear mechanism is also what is called a single-pinion planetary gear mechanism including three elements, that is, a fourth sun gear, a fourth ring gear, and a fourth carrier that supports a fourth pinion so that the fourth pinion is rotatable about its axis and revolvable. The fourth pinion engages with the fourth sun gear and the fourth ring gear. When the three elements of the fourth planetary gear mechanism are defined as a tenth element, an eleventh element, and a twelfth element that are arranged from one side in the sequence shown in the collinear diagram of the fourth planetary gear mechanism, the tenth element is the fourth ring gear, the eleventh element is the fourth carrier, and the twelfth element is the fourth sun gear.
The first sun gear (first element) of the first planetary gear mechanism is connected to the input shaft. The fourth ring gear (tenth element) of the fourth planetary gear mechanism is connected to the output member.
The first carrier (second element) of the first planetary gear mechanism is connected to the second carrier (fifth element) of the second planetary mechanism and the third ring gear (ninth element) of the third planetary gear mechanism, and form a first connected member (second element, fifth element, and ninth element). The first ring gear (third element) of the first planetary gear mechanism and the fourth sun gear (twelfth element) of the fourth planetary gear mechanism are connected to each other, and form a second connected member (third element and twelfth element). The third carrier (eighth element) of the third planetary gear mechanism and the fourth carrier (eleventh element) of the fourth planetary gear mechanism are connected to each other, and form a third connected member (eighth element and eleventh element).
The automatic transmission in Patent Document 1 includes a total of six engagement mechanisms including three clutches, that is, first to third clutches, and three brakes, that is, first to third brakes.
The first clutch is a wet multi-plate clutch and is configured so as to be switchable between a connection state in which the sun gear (first element) of the first planetary gear mechanism and the third connected member (eighth element and eleventh element) are connected to each other and a release state in which the sun gear and the third connected member are disconnected from each other. The second clutch is a wet multi-plate clutch and is configured so as to be switchable between a connection state in which the first sun gear (first element) of the first planetary gear mechanism and the second ring gear (fourth element) of the second planetary gear mechanism are connected to each other and a release state in which the first sun gear and the second ring gear are disconnected from each other.
The third clutch is a wet multi-plate clutch and is configured so as to be switchable between a connection state in which the second sun gear (sixth element) of the second planetary gear mechanism and the second connected member (third element and twelfth element) are connected to each other and a release state in which the second sun gear and the second connected member are disconnected from each other. The first brake is a wet multi-plate brake and is configured so as to be switchable between a fixed state in which the third connected member (eighth element and eleventh element) is fixed to the housing and a release state in which the third connected member is unfixed from the housing.
The second brake is a wet multi-plate brake and is configured so as to be switchable between a fixed state in which the third sun gear (seventh element) of the third planetary gear mechanism is fixed to the housing and a release state in which the third sun gear is unfixed from the housing. The third brake is a wet multi-plate brake and is configured so as to be switchable between a fixed state in which the second sun gear (sixth element) of the second planetary gear mechanism is fixed to the housing and a release state in which the second sun gear is unfixed from the housing.
In the automatic transmission in Patent Document 1, a forward first gear is established by setting the first brake, the second brake, and the third brake in a fixed state. A forward second gear is established by setting the second brake and the third brake in the fixed state and the third clutch in a connection state. A forward third gear is established by setting the second brake and the third brake in the fixed state and the second clutch in a connection state. A forward fourth gear is established by setting the second brake in the fixed state and the second clutch and the third clutch in the connection state.
A forward fifth gear is established by setting the second brake in the fixed state and the first clutch and the second clutch to the connection state. A forward sixth gear is established by setting the three clutches, that is, the first to third clutches, in the connection state. A forward seventh gear is established by setting the third brake in the fixed state and the first clutch and the second clutch to the connection state. A forward eighth gear is established by setting the third brake to the fixed state and the first clutch and the third clutch in the connection state. A reverse gear is established by setting the first brake and the third brake in the fixed state and the second clutch in the connection state.